(Just for the sake of completeness, rather than a warning against microwaving)
I seem to remember reading a few years ago (I think it was Int. J. Peptide
Protein Res., but I don't have the reference to hand) that microwaving
solutions of amino acids would cause a degree of racemisation. This
probably involved a strange solvent or an extreme of microwaving but
would show that you *can* cause structural changes. I do not know if such
changes could occur in proteins and I doubt if they would be be any cause for
concern with respect to warming up milk.
Also, one person mentioned sterilising plastic tubes in a microwave. I
assume that the beaker enclosing them had some water in it as microwaving
empty plastic tubes (dry) could prove rather nasty. I also find that, for
sterilisation of relatively small amounts of liquid, less than a minute
in the microwave is sufficient - not even enough to boil the solution.
After all, its not the heat that is needed to kill bacteria/yeast etc.
More power to your microwaves,
Bernard
Bernard Murray, Ph.D.
bernard at elsie.nci.nih.gov (National Cancer Institute, NIH, Bethesda MD, USA)